The Changing Clyde

One of Scotland’s most remarkable marine ecosystems — a place of extraordinary natural diversity, profound environmental change and growing opportunities for recovery.

The Firth of Clyde is much more than the stretch of sea separating Scotland’s west coast from its islands.

It is a complex marine ecosystem, shaped by powerful geological forces, changing ocean conditions and centuries of human activity.

From the deep, sheltered sea lochs of Argyll to the waters surrounding Arran, Bute and the Cumbraes, the Clyde encompasses a remarkable variety of underwater landscapes and habitats.

These waters support fish, seabirds, seals, harbour porpoises, dolphins and visiting whales, together with an extraordinary diversity of life that remains largely hidden beneath the surface.

Yet the Clyde has also undergone substantial environmental change.

A long history of industrial development, intensive fishing, shipping, coastal construction and pollution has influenced the structure and functioning of its marine ecosystem.

Today, the Clyde faces the additional challenges of climate change and increasing competition for marine space.

Understanding these changes — and how different pressures interact — is essential if we are to protect and restore the natural systems on which marine wildlife and coastal communities depend.

A Remarkable Marine Landscape

The Clyde’s distinctive geography is central to its ecological importance.

Much of its underwater landscape was shaped during the last ice age, when glaciers carved deep valleys that later flooded as sea levels rose.

These valleys formed the sheltered sea lochs that characterise much of the upper Clyde and Argyll coast.

In places such as Loch Goil, Loch Long and Loch Fyne, steep underwater cliffs descend into deep basins containing fine mud and specialised marine communities.

Some of these habitats support animals more commonly associated with much deeper offshore waters.

Elsewhere, tidal currents flowing around headlands, through narrow channels and across shallow sills create highly variable conditions.

These differences in depth, seabed type, salinity and water movement contribute to the Clyde’s remarkable biodiversity.

The Living Seabed

The seabed is a vital part of the marine ecosystem.

Habitats such as seagrass meadows, maerl beds, kelp forests, flame shell beds and burrowed mud communities support a wide variety of marine animals.

Some provide shelter for juvenile fish, while others support the invertebrates and smaller species that form important parts of marine food webs.

The deep mud habitats of the Clyde’s sea lochs can support sea pens, burrowing crustaceans and other specialised animals.

These habitats are not simply features of the seabed. They help sustain the wider ecosystem, including the fish populations upon which marine predators depend.

Protecting them is therefore closely connected to protecting the Clyde’s whales, dolphins, porpoises and seabirds.

A Sea Shaped by Human History

For centuries, the Clyde has been central to Scotland’s maritime history.

Fishing, shipbuilding, engineering, shipping and international trade helped shape the communities surrounding its shores.

The Firth was once renowned for productive herring fisheries and substantial populations of commercially important fish.

However, the twentieth century brought major changes to the fishing industry and the wider marine environment.

The expansion of mobile fishing gear, changes in fishing effort and other environmental influences contributed to substantial alterations in the composition of fish communities.

Research examining scientific survey records from 1927 to 2009 documented a striking transformation in the Clyde’s demersal fish community.

Larger fish became much less prominent, while the biomass increasingly consisted of smaller species, particularly whiting.

Although the ecosystem remained productive, its structure had changed considerably.

This distinction is important.

An ecosystem can continue producing large quantities of marine life while becoming fundamentally different from the ecosystem that existed in the past.

Understanding whether marine ecosystems are recovering therefore requires more than measuring the quantity of fish present. We must also consider the diversity, size structure and ecological roles of the species they contain.

Fishing and the Wider Food Web

Fishing remains an important economic and cultural activity in the Clyde.

However, different fishing methods can have very different ecological consequences.

Bottom-towed fishing gears, including certain trawls and dredges, can physically disturb seabed habitats and affect the animals living within them.

Repeated disturbance can alter habitat complexity and influence the communities of fish and invertebrates that depend on the seabed.

Fishing can also affect marine mammals indirectly through changes in prey populations.

Harbour porpoises depend on a reliable supply of suitable fish. Changes in prey availability may influence where they forage, how much energy they expend and which habitats they use.

Static fishing gear presents a different concern.

Gillnets and tangle nets can accidentally entangle harbour porpoises, while ropes associated with fishing equipment can pose an entanglement risk to larger whales.

The challenge is to develop fisheries management that recognises these different pressures while supporting sustainable fishing opportunities and coastal livelihoods.

Pollution and the Legacy of Industry

The Clyde’s industrial history has left a lasting environmental legacy.

During periods of intensive industrial activity, contaminants entered the marine environment through industrial discharges, wastewater and other sources.

Some hazardous substances, including heavy metals and persistent organic pollutants, can remain in marine sediments long after their original release.

Although regulation and improvements in wastewater treatment have reduced many historical sources of pollution, legacy contamination remains a consideration in parts of the Clyde.

Pollutants can affect marine life directly and may accumulate through food webs.

For predators such as harbour porpoises and dolphins, this is particularly important.

Research elsewhere in UK waters has identified associations between persistent chemical contamination and reproductive problems in harbour porpoises.

The condition of marine mammals can therefore provide useful evidence about environmental pressures that may not be immediately visible from the surface.

At the same time, improvements in environmental regulation and water quality demonstrate that damaging human impacts need not be permanent.

Recovery is possible when environmental problems are recognised and addressed.

A Busy and Increasingly Noisy Sea

The Clyde is a working marine environment.

Commercial shipping, ferries, naval activity, fishing vessels, recreational boating, aquaculture and coastal construction all contribute to its maritime character.

These activities provide important economic and social benefits, but they also introduce pressures into the marine environment.

One of the least visible is underwater noise.

Listening Beneath the Surface

For whales, dolphins and porpoises, sound is fundamental to life underwater.

Harbour porpoises rely on high-frequency echolocation to locate prey and navigate their surroundings.

Dolphins use a variety of clicks and whistles for echolocation and social communication.

Other cetaceans produce sounds that can travel considerable distances through seawater.

Human activities introduce additional sounds into this environment.

Shipping produces predominantly lower-frequency noise, while some sonar systems, acoustic deterrent devices and marine construction activities can generate more intense or higher-frequency signals.

The effects depend on the characteristics of the sound, its duration, the distance from the source and the hearing sensitivity of the animal.

Potential consequences include behavioural disturbance, changes in feeding activity, displacement from habitat and, at sufficiently high exposures, hearing damage.

Not every underwater sound is harmful, but understanding the acoustic environment is essential to assessing the conditions experienced by marine mammals.

Clyde Porpoise has undertaken investigations into underwater sound from acoustic deterrent devices, sonar and other sources, contributing to a better understanding of the pressures potentially affecting cetaceans in the region.

Climate Change and an Uncertain Future

Climate change adds another dimension to the environmental pressures facing the Clyde.

Warming seas, changes in rainfall and freshwater input, shifts in marine productivity and ocean acidification can influence marine ecosystems.

Some species may change their distribution as water temperatures and environmental conditions alter.

Others may be affected indirectly through changes in prey availability or the habitats on which they depend.

For cetaceans, these changes may influence feeding opportunities, seasonal movements and habitat suitability.

The consequences will not necessarily be the same for every species.

Some may benefit temporarily from changing prey distributions, while others could face increasing ecological challenges.

Understanding these responses requires sustained monitoring over many years.

Importantly, climate change does not occur in isolation. It may interact with existing pressures from fishing, pollution, habitat disturbance and underwater noise.

When Environmental Pressures Overlap

Perhaps the greatest challenge facing the Clyde is understanding the combined effects of different human activities.

Environmental assessments often examine activities individually, yet marine animals experience their environment as a whole.

Consider a harbour porpoise feeding in one of the Clyde’s sea lochs.

The animal may occupy an area where commercial vessel traffic, aquaculture, fishing and recreational boating overlap.

Its prey may be influenced by changes in seabed habitats or wider environmental conditions.

At the same time, underwater sound could temporarily interrupt feeding activity or affect how the animal uses its habitat.

An individual disturbance may be relatively brief, but repeated disturbances could have greater consequences, particularly if they occur in an important feeding area.

This is the principle of cumulative environmental impact.

The combined effects of different pressures are not necessarily equal to the simple sum of their individual impacts.

Some pressures may reinforce one another, while others may operate independently or affect animals at different times.

Understanding cumulative impacts requires information about the distribution of wildlife, the locations and timing of human activities, and the biological consequences of exposure.

This is particularly important for highly mobile marine mammals, which may encounter several different pressures as they move between feeding areas.

Important Habitat Does Not Always Mean Undisturbed Habitat

An important consideration is that the continued presence of marine mammals does not necessarily indicate a healthy or undisturbed environment.

Animals may continue using productive feeding grounds despite exposure to considerable human activity.

Where prey resources are particularly valuable, animals may have limited opportunities to avoid disturbance without sacrificing feeding opportunities.

Consequently, the presence of harbour porpoises, dolphins or whales should not automatically be interpreted as evidence that human activities are having no effect.

We need to understand not just where animals occur, but how they behave, feed and respond to the conditions around them.

Protecting and Restoring the Clyde

Despite its history of environmental change, the Clyde retains considerable ecological value and opportunities for recovery.

Marine Protected Areas have been established to help safeguard important seabed habitats and species.

These include the South Arran Marine Protected Area, the Upper Loch Fyne and Loch Goil Marine Protected Area and the Clyde Sea Sill Marine Protected Area.

Their protected features include valuable habitats and ecological processes, such as maerl beds, flame shell beds, burrowed mud communities and tidal fronts.

Marine restoration initiatives are also contributing to efforts to recover habitats and species that have declined.

These include projects involving native oysters and other components of coastal ecosystems.

However, effective conservation depends on more than designating protected areas.

It also requires appropriate management, monitoring, enforcement and an understanding of how activities outside protected boundaries may influence the habitats and species within them.

Restoring the Clyde means considering the ecological connections between its seabed, fish populations, marine mammals and coastal communities.

Marine Spatial Planning — Looking at the Bigger Picture

The Clyde is used for many different purposes, and its management involves a wide range of interests.

Marine spatial planning offers an opportunity to consider these uses together rather than treating each activity in isolation.

The Clyde Marine Planning Partnership brings together organisations and stakeholders involved in the future management of the Clyde Marine Region.

A key challenge is ensuring that environmental protection, economic activity and community interests are considered within a coordinated approach.

Clyde Porpoise has contributed to marine planning discussions, advocating for greater recognition of cetacean habitats, improved consideration of cumulative pressures and the inclusion of community-based scientific evidence in decision-making.

Our involvement reflects a belief that marine conservation should be informed by scientific research, practical experience and meaningful participation by the communities connected to the sea.

Effective marine planning must recognise not only how people use the Clyde, but how marine wildlife uses the same waters.

What Can Harbour Porpoises Tell Us?

Harbour porpoises offer a valuable perspective on the changing Clyde.

As predators dependent on fish populations, their distribution and feeding behaviour are closely linked to the marine food web.

Their reliance on high-frequency echolocation also makes them particularly relevant to investigations of underwater sound.

By studying porpoises, we can begin to examine how changes in habitat, prey availability and human activity may influence the conditions experienced by marine mammals.

However, porpoises cannot provide a complete assessment of ecosystem health on their own.

Their observations and acoustic recordings must be considered alongside information on fish populations, seabed habitats, pollution, water quality and other environmental factors.

It is through combining these different sources of evidence that a clearer picture of the Clyde’s ecological condition can emerge.

Clyde Porpoise — Building the Evidence

Since 2016, Clyde Porpoise has undertaken vessel-based passive acoustic surveys and related investigations into harbour porpoise ecology and the underwater environment.

Our historical acoustic recordings provide opportunities to examine porpoise occurrence, investigate behaviour and develop more robust methods for estimating distribution and abundance.

We are also exploring how information on marine mammal activity can be combined with environmental observations and the spatial distribution of human pressures.

Some of this work remains under development, and findings require careful validation before firm conclusions can be drawn.

Nevertheless, long-term monitoring offers an important advantage: the opportunity to revisit earlier observations and compare them using improved scientific methods.

Historical acoustic recordings cannot be collected again.

Their value may therefore increase as we seek to understand how the Clyde’s marine environment changes over time.

Alongside our scientific research, Clyde Porpoise continues to contribute to community conservation, marine planning and discussions concerning the protection and recovery of Scotland’s inshore waters.

A Future Worth Protecting

The Clyde’s history demonstrates both the scale of environmental change that can occur and the continuing resilience of marine ecosystems.

Its waters remain biologically productive and support remarkable wildlife, from harbour porpoises feeding in sheltered sea lochs to visiting humpback whales and the extraordinary communities inhabiting its seabed.

There are genuine opportunities to improve the condition of marine habitats, strengthen environmental protection and develop more sustainable relationships between human activity and the natural environment.

Realising these opportunities requires reliable scientific evidence, long-term monitoring, effective management and collaboration.

For Clyde Porpoise, understanding the changing Clyde is central to our work.

Our ambition is to contribute knowledge that helps identify important marine mammal habitats, investigate emerging environmental pressures and support decisions that recognise the ecological connections sustaining the wider marine ecosystem.

The challenge is not simply to conserve the wildlife that remains, but to understand, protect and restore the ecological processes that will allow the Clyde’s marine environment to thrive in the future.